1. Field of the Invention
The present invention relates to an optical fiber and an optical-fiber transmission line for a long-haul optical signal transmission.
2. Description of the Related Art
For example, an optical transmission line is used that is formed by combining a large-Aeff-type nonzero dispersion-shifted optical fiber having a large effective core area and a low-slope-type nonzero dispersion-shifted optical fiber having a small dispersion slope, as an optical transmission line for undersea installation (see, e.g., M. Suzuki, et al., “Dispersion-Managed High-Capacity Ultra-Long-Haul Transmission”, J. Lightwave Technol., vol. 21, no. 4, pp. 916-929, April 2003). A “nonzero dispersion-shifted optical fiber” is a single-mode optical fiber that has, at a wavelength of a signal light, for example, an extremely small wavelength dispersion of about −1 ps/nm/km to −5 ps/nm/km or about 1 ps/nm/km to 5 ps/nm/km. In an undersea transmission line, nonzero dispersion-shifted optical fibers each having negative dispersion are often used.
Optical characteristics of the large-Aeff-type nonzero dispersion-shifted optical fiber are, for example, 75 μm2 as the effective core area and 0.10 ps/nm2/km as the dispersion slope. Optical characteristics of the low-slope-type nonzero dispersion-shifted optical fiber are, for example, 50 μm2 as the effective core area and 0.05 ps/nm2/km as the dispersion slope. The average optical characteristics of an optical-fiber transmission line formed by connecting the above optical fibers both having the substantially same length are 65 μm2 as the effective core area and 0.07 ps/nm2/km as the dispersion slope.
Usually, in the nonzero dispersion-shifted optical-fiber transmission line, an optical signal is transmitted from the large-Aeff-type nonzero dispersion-shifted optical fiber side. As a result, in a state where the optical intensity of the optical signal is high, occurrence of non-linear optical phenomena is suppressed because the effective core area of the optical transmission line is large. Thereafter, the optical signal is input into the low-slope-type nonzero dispersion-shifted optical fiber after the optical intensity is attenuated due to the transmission loss of the optical fiber. The low-slope-type nonzero dispersion-shifted optical fiber has a relatively small effective core area. However, the fiber has a small dispersion slope and the difference between the wavelength dispersions for different wavelengths is small. As a result, for transmission of a wideband WDM signal light formed by wavelength-division-multiplexed signal lights respectively having different wavelengths, generation of any deviation of wavelength dispersion between the optical signals is suppressed.
That is, in the nonzero dispersion-shifted optical fiber, the effective core area and the dispersion slope are in a tradeoff relationship. Therefore, moderation of the tradeoff relationship is facilitated for the entire optical-fiber transmission line by configuring the optical transmission line by combining the large-Aeff-type nonzero dispersion-shifted optical fiber and the low-slope-type nonzero dispersion-shifted optical fiber.
A technique using a multimode optical fiber is disclosed as a technique of significantly expanding the effective core area by moderating the above tradeoff relationship (see, e.g., Japanese Patent Application Laid-open No. 2004-271904).
Because all of the above nonzero dispersion-shifted optical fibers each have a negative wavelength dispersion at the wavelength of the signal light, a negative accumulated wavelength dispersion is generated for the entire optical-fiber transmission line. Therefore, the dispersion needs to be compensated using a dispersion compensating optical fiber that has a positive wavelength dispersion at the wavelength of the signal light. Conventionally, this type of dispersion compensating fiber has the same configuration as that of a standard single-mode optical fiber having the zero-dispersion wavelength of about 1310 nanometers, and uses a cutoff shift optical fiber of which the bending resistance in the fundamental propagation mode thereof is enhanced by shifting the cutoff wavelength thereof to 1550 nanometers.
However, all of the large-Aeff-type and the low-slope-type nonzero dispersion-shifted optical fibers and a cutoff shift optical fiber each have a positive dispersion slope. Therefore, even when wavelength dispersion of an optical-fiber transmission line at the wavelength of 1550 nanometers is compensated using, for example, a cutoff shift optical fiber, compensation of the dispersion slope can not be executed. As a result, at wavelengths other than 1550 nanometers for which the dispersion is compensated, accumulated wavelength dispersion remains, and the remaining accumulated wavelength dispersion becomes larger as the wavelength becomes away from 1550 nanometers. Therefore, a problem has arisen that a broadband WDM transmission is not enabled.